The present invention relates to a thin film temperature sensing device, for example a thin film thermocouple, which can be readily incorporated into a cutting tool.
With traditional machining operations, a significant amount of heat is generated during the process of cutting a material ("work piece") with a cutting tool. The majority of the heat produced that goes into the cutting tool is due to friction between a chip which is produced by the cutting tool and the top of the cutting tool. A small amount of heat due to friction is also generated between the tip of the cutting tool and the work piece. Thus, typically the hottest spot ("hot spot") of the working tool is not on the cutting edge of the tool, but at the top of the tool.
The heat generated is an important limiting factor with conventional machine tools. The heat can raise the temperature of the cutting tool to a point where the tool's mechanical and material characteristics become undesirable for cutting. For example, there is a critical temperature for the tool above which there is a rapid decrease in mechanical strength. The life of the cutting tool is defined as the period of time for which the tool can cut at a desired rate and produce a desired quality. Both of these quantities are dependent on the sharpness of the tool. Tool sharpness decreases as the frictional wear on the tool progresses. Tool degradation, or wear, occurs more rapidly as the temperature of the tool increases.
It has become increasingly important in the field to produce and develop cutting tools that can survive at higher cutting rates. Thus, designers have been developing new materials for cutting tools that can survive the higher temperatures produced at the desired higher cutting rates. For example, the evolution of the cutting tool has progressed from tool steel, to high speed steel (HSS), to cast alloys, to cemented carbides, to ceramics, and to diamond. HSS is the most widely used material because it is the strongest of the cutting materials, and therefore the most rugged.
It has also become increasingly popular in the industry to coat cutting tools with a thin ceramic coating to provide a hard, non-reactive, and smooth surface. The thin ceramic coatings are not as brittle as their solid counter-parts and provide increased benefits in hardness and smoothness. These coatings have greatly improved the wear resistance of tools.
Although single coating of cutting tools is the most commonly accepted practice today, multiple coating of the tools is gaining wider acceptance. The additional coatings in essence provide a battery of multiple thermal boundaries at the interfaces between the coatings. The temperature drops several degrees across each interface. If multiple coatings are used, the effect is cumulative and the net result is a very effective thin thermal insulation for the cutting tool. These multiple coatings have been successfully used to increase the cutting rates of the tools.
While tool designers have been developing new materials and coatings to improve productivity, machine designers have been developing computer control systems that allow the cutting operations to occur at optimal conditions. The two most common computer technologies are CNC control and statistical tool control. CNC control allows the operator to specify the cutting parameters through a series of numbers that can be stored in a computer file. Each time the file is accessed and executed, the computer will cause the machine to cut exactly the same way. Ideally, one would identify the set of numbers that would allow optimal cutting to occur, but in fact most manufacturing occurs at only 20 to 80 percent of optimal capacity. The optimal parameters for a part with complex and variable geometry are difficult to determine.
CNC Technology is static in that the cutting parameters do not change during execution of the program file. The specified parameters are based on a presumption that the tool's characteristics will not change. However, this presumption generally is not true. Tools tend to dull and their cutting characteristics will vary over time. In order to compensate for these variations, one could write an equation into the file program to estimate the changes, or develop a sensor to track conditions indicative of the changes. Thus, a sensing system which would allow an operator or computer to dynamically monitor the variation in conditions would be a significant advancement in the industry.
Statistical tool control is based on the concept that if you cut exactly the same way each time, a cutting tool will last on average the same period of time. Manufacturers typically collect the cutting life of several dozen tools that cut the same family of parts to determine the statistical average tool life. However, two major problems exist in this method. There is a significant amount of variability in the life of each tool and the statistical average is based on a high survival rate. Unfortunately, some tools have much shorter lives than others which significantly impacts upon the variability and statistical average. Since the cost of having the tool fail while in operation generally far exceeds the cost of replacing the tool before failure, the manufacturers replace the tools based on the shortest cutting tool life.
As a result, most manufacturing today is operating at far below optimal conditions. This problem is acutely pronounced in modern high speed cutting systems. High speed cutting systems offer greater productivity, but put the cutting tool at higher risk of temperature damage. Thus, manufacturers intentionally program the CNC machine to operate at speeds below optimal. Thus, ideally one would desire a measurement or indication that can be correlated to the optimal cutting conditions of the tool and that can be used as feedback for controlling the feed rates and speeds of the cutting process. Temperature of the cutting tool is such an indication.
Direct measurement of the cutting tool temperature is preferred and several attempts have been made in this regard. Unfortunately, direct measurement of the cutting tool temperature is complicated by a number of factors. For example, the use of coolant, the sheer abrasion of the removed chips, and a sharp temperature gradient within the cutting tool are examples of such factors. The use of coolant obstructs the cutting edge from view and renders non-contact temperature measurement techniques, such as infrared optical pyrometery, extremely difficult. Additionally, the use of coolant makes the temperature gradient sharper and less predictable since the flow of coolant is not necessarily uniform and the flow rate is not constant. Other direct measuring techniques such as placing a temperature sensor on top of the cutting tool are ineffective because the abrasion of metal chips literally pushes the device off of the tool. Indirect measuring techniques have been suggested but, these techniques have failed to provide the necessary responsiveness needed for computer control, nor the ruggedness needed to survive the cutting process.
Thin film thermocouple sensors have been developed and used in industry in various applications. For example, the scholarly article entitled "Thermal and Sputtered Aluminum Oxide Coatings for High Temperature Electrical Insulation," by Kenneth G. Kreider and Stephen Semancik, J. Vac. Sco. Technol. A3(6), November/December 1985, describes the use of thin film sensors to provide temperature measurements of critical areas of turbine engine blades and vanes. The article "Thin Film Thermocouples for Internal Combustion Engines," by Kenneth G. Kreider; J. Vac. Sco. Technol. A4(6), November/December 1986, describes the use of such devices for thermometry of internal combustion engine parts including valves, valve seats, combustion chamber walls, and piston heads. However, conventional applications of thin film sensors have been in non-abrasive environments. Typically, thin film thermocouples have been considered as relatively delicate device and not suitable for a physically abusive environment, such as with cutting tools.